The invention relates to an optical-chemical sensor which is suitable for the continuous and discontinuous determination by luminescence optics of the concentration of chloride in an aqueous sample and which comprises a luminescence indicator and a polymer carrying the luminescence indicator, as well as to a process for the production of such an optical-chemical sensor.
Chloride is the mainly anionic substance of extracellular liquids in the body and plays an important role in maintaining the distribution equilibrium of water, the osmotic pressure and the equilibrium of anionic and cationic constituents. The normal values of chloride in the human blood serum are approximately 97-108 mmol/l; the total concentration of anionic substances is approximately 154 mmol/l. Pathological chloride valuesxe2x80x94of up to 170 mmol/lxe2x80x94are found in the case of osmotic diuresis plus insufficient water supply, when the mechanism of the thirst regulation is disturbed, in the case of nephrogenic diabetes insipidus and in the case of enteral bicarbonate losses. Minimum valuesxe2x80x94of down to 30 mmol/lxe2x80x94are found in the case of gastro-intestinal chloride losses and are associated with the chronic pyelonephritis (Addison""s disease) and renal failure. Therefore, chloride is the halide which in clinical diagnostics has to be determined most frequently.
Besides titrimetric and photometric determination methods, which are very accurate but also involve much work, potentiometric methods with ion-selective electrodes have become generally accepted in practice. The disadvantages are the minor specificity of chloride-selective electrodes, their sensitivity to proteins and the need for a reference electrode.
Recently, a series of optical sensors having indicator systems partly working in different manners have been proposed for the chloride determination. The so-called coextraction optode (EP 0 358 991) is based on a lipophile pH indicator dye with a lipophile counterion, is very sensitive to lipophile anionic constituents of the sample and only provides useful results when the pH value is very exactly known.
It is known that both the luminescence intensity and the luminescence decay time of certain luminescence indicators are diminished by dynamic luminescence quenching of halide ions. This is expressed by the Stern-Vollmer equation:             F      0        F    =            1      +                        k          q                ·                  τ          0                ·                  [          Q          ]                      =          1      +                        K          SV                ·                  [          Q          ]                    
In this equation, F0 and F stand for the relative luminescence intensities in the absence and presence of a quencher, [Q] for the concentration of the quencher, kq for the bimolecular rate constant for the quenching, xcfx840 for the lifetime of the excited state in the absence of a quencher and KSV for the Stern-Vollmer quenching constant. Thus, it is possible to deduce the chloride concentration of the sample from the relative luminescence intensity and/or the luminescence decay time of the luminescence indicator.
A chloride-sensitive optode on the basis of a quinoline or acridine dye which is covalently linked to a glass surface and the luminescence of which is quenched as a function of the halide concentration was proposed in AT-B 384 891.
Chloride-sensitive luminescence indicators having quaternized heteroaromatic N atoms have a low pH cross-sensitivity and a low cross-sensitivity to physiological concentrations of disturbing ions. However, when it comes to commercial applications involving very large numbers, the disadvantages are the absorption wavelengths of  less than 450 nm which are in the ultraviolet spectral range (not accessible with blue LEDs that are commercially available at a moderate cost) and the chemical immobilization on the surfaces of suitable transparent carrier materials, which is cumbersome in particular when dealing with large numbers.
For the investigation of the chloride transport and of regulation mechanisms in isolated membrane vesicles, reconstituted liposomes and living cells and tissues by luminescence measuring, a series of quinoline and acridine derivatives and lucigenine (a bisacridine) are commercially available (Richard P. Haughland, xe2x80x9cHandbook of Fluorescent Probes and Research Chemicalsxe2x80x9d, 6th edition, pp. 577-579).
A chloride-sensitive optical-chemical sensor on the basis of a 3,6-bis(dimethylamino)-acridine being present in a polyacrylamide layer and having a lipophile aliphatic hydrocarbon chain with up to 30 C atoms is described in U.S. Pat. No. 5,691,205. This indicator can be excited by a blue light source (LED) at 488 nm.
The production of this known sensor comprises the productionxe2x80x94at the end of a light-conducting fiberxe2x80x94of a thin membrane or layer consisting of polyacrylamide by photopolymerization of a monomer solution consisting of acrylamide-N,Nxe2x80x2-methylenebis(acrylamide), riboflavin and ammonium peroxodisulfate. Subsequently, the membrane or layer is immersed into an indicator solution, the lipophile indicator diffusing into the membrane or layer. U.S. Pat. No. 5,691,205 does not give information on stability properties of that sensor, particularly the leaching property of the indicator in the case of a quite long contact with measuring liquids, such as blood.
In the case of the sensor known from U.S. Pat. No. 5,691,205, the disadvantages are, particularly with a view to the production of large numbers of constant quality, the manufacturing step of the membrane or layer by photopolymerization of a monomer solution and the covering of the membrane or layer with an indicator. With regard to a constant quality of the sensors, this step is very cumbersome.
The present invention has as its object to provide an optical-chemical sensor which is suitable for the determination by luminescence optics of the concentration of chloride in an aqueous sample and which does not have the above-indicated disadvantages. In particular, it should be possible to excite the sensor by commercially available LEDs (excitation wavelength greater than 460 nm), to manufacture very large numbers thereof at a moderate cost and in a reproducible way and, preferably, to use it for the determination of physiological chloride concentrations.
In an optical-chemical sensor which is suitable for the continuous and discontinuous determination by luminescence optics of the concentration of chloride in an aqueous sample and which comprises a luminescence indicator and a polymer carrying the luminescence indicator, the object of the invention is achieved in that the luminescence indicator is a non-lipophile acridine or bisacridine compound and the polymer is a linear-chain hydrophile polymer soluble in an organic solvent. The term of xe2x80x9clinear-chainxe2x80x9d should express that the polymer is not cross-linked.
It is obvious that the polymer should not be soluble substantially in the sample, e.g. blood, sea water, salt-containing aqueous liquids.
The advantages of such a sensor for measuring chloride are
a wide dynamic measuring range, in particular in the physiologically relevant concentration range of chloride;
the high sensitivity;
the high stability and reproducibility;
the high selectivity for chloride; and
a low pH cross-sensitivity in the physiologically relevant pH-value range.
Preferably, the acridine or bisacridine compound is selected from a group comprising methylacridinium methosulfate (MAC), 4-nitrophenylbutylacridinium methosulfate (NPBA), N,Nxe2x80x2-di-(3-sulfopropyl)-9,9-bisacridinium (SPBA), N,Nxe2x80x2-diacetic acid ethyl ester-9,9-bisacridinium (AEBA) and lucigenine.
As polymer, ion-permeable multiple block copolymers containing acid amide and nitrile and/or acid imide and/or carboxylate groups are preferred.
For example, such a linear-chain hydrophile polymer is commercially available multiple block copolymer HYPAN, available from HYMEDIX Int. Inc., Dayton, N.J., which will be mentioned in more detail below. It is decisive that this polymer can be dissolved in an organic solvent such as DMSO, wherein it is likewise easy to evaporate the solvent after the solution has been applied to a transparent carrier material, so that a simple and reproducible production of a sensor is enabled.
Multiple block copolymers that can be used in accordance with the invention are characterized in that each polymer chain consists of several sequences of units having hydrophile properties, e.g. independently of each other acid amide groups which also may be replaced by hydrophile groups, acid imide groups, carboxylate groups and several sequences having groups of high cohesive energy, e.g. nitrile groups. The production of such polymers is described for example in U.S. Pat. Nos. 4,331,783 and 4,379,874.
Surprisingly, it has been found that the above-named acridine and bisacridine compounds diffuse into the polymer when the linear-chain hydrophile polymer is immersed into a solution of these compounds and can only be leached slowly afterwards. Thus, the present invention provides a very stable sensor.
Among the above-indicated acridine and bisacridine compounds, lucigenine is particularly suitable for the determination of physiological chloride concentrations in blood, serum and plasma thanks to the following properties:
(a) high quenching constant KSV in solution (KSV (Clxe2x88x92) greater than 100 Mxe2x88x921);
(b) excitation maximum above 450 nm, so that an inexpensive LED may be used as a light source;
(c) large Stokes shift;
(d) high photostability;
(e) luminescence quantum yield higher than 0.5; and
(f) commercial availability.
According to a preferred embodiment of the sensor according to the invention, the polymer carrying the luminescence indicator is applied in the form of a film to a transparent carrier material. Here, the film preferably has a thickness of up to 20 xcexcm. It is essential that the film is not soluble in aqueous liquids, i.e. in the sample or the calibration liquids.
According to another preferred embodiment, the polymer carrying the luminescence indicator is embedded in the form of fine particles in a hydrogel film applied to a transparent carrier material. Here, the size of the particles can be up to 20 xcexcm and preferably is  less than 1 xcexcm. By this particularly advantageous embodiment, large quantities of luminescence-indicator-carrying material can be manufactured in one manufacturing step at a moderate cost. Another essential advantage is that with this material, large quantities of sensors can be produced, which have particularly uniform characteristics, such as characteristic curve and luminescence-indicator load.
Hydrogels used in accordance with the invention are ion-permeable hydrophile polymers insoluble in water or aqueous salt solutions. Preferably, they should be soluble in organic solvents or solvent mixtures (e.g. EtOH/H2O) in which the polymer carrying the luminescence indicator is insoluble, so that the solvent can be evaporated after the application of the layer. Hydrogels preferably used are the linear-chain hydrophile polymers of Tyndale Plains-Hunter (see below), for example. In principle, it is also possible to use other hydrogels, e.g. cross-linked polymers, in particular polyacrylamide, the monomer solution being polymerized after the application.
Preferably, an additional layer is applied to the film or hydrogel film that is on the transparent carrier material, which layer is composed of a hydrophile, ion-permeable polymer and preferably contains color pigments, in particular black pigments. Advantageously, this additional layer brings about an optical uncoupling of the sensor from the measuring medium. It acts as an ion-permeable optical isolating layer.
According to another embodiment, the transparent carrier material is a light-conducting fiber, the film carrying the luminescence indicator being applied to a light-conducting fiber.
The invention also relates to a process for the production of an optical-chemical sensor which is suitable for the continuous and discontinuous determination by luminescence optics of the concentration of chloride in an aqueous sample and which comprises a luminescence indicator and a polymer carrying the luminescence indicator, characterized in that
a non-lipophile luminescence indicator on the basis of an acridine or bisacridine compound is used as luminescence indicator and
a linear-chain hydrophile polymer soluble in an organic solvent is used as polymer, wherein
a solution of the luminescence indicator is produced and
the solution is brought into contact with the linear-chain hydrophile polymer, the luminescence indicator diffusing into the linear-chain hydrophile polymer.
According to a preferred embodiment, the luminescence indicator diffused into the linear-chain hydrophile polymer is immobilized by radiation, preferably with ultraviolet and/or blue light (320 nm to 490 nm).
When using a multiple block copolymer of the HYPAN type, it was surprisingly found that particularly acridines and bisacridines having at least one methyl group on the quaternized N atom (e.g. MAC, lucigenine) are not leached after radiation with blue and/or ultraviolet light, even in the case of a quite long contact with measuring liquids. This leaching incapacity is essential particularly in measurements performed in a continuous manner (xe2x80x9cmonitoringxe2x80x9d) and in the case of a multiple use of sensors.
Experiments established for example that with HYPAN HN80 radiated with blue or ultraviolet light and carrying MAC (FIG. 1) or lucigenine, the solvent liberated from the polymer was clear and the polymer fraction still had the typical yellow coloring of the indicator after the polymer had been dissolved in DMSO and the solution had been centrifugated under addition of MeOH. A corresponding control experiment with an indicator-carrying polymer not radiated with blue or ultraviolet light established that the solvent liberated from the polymer had the typical yellow coloring of the indicator and the polymer fraction was uncolored. This experiment suggests that the radiation of the indicator-carrying polymer with blue or ultraviolet light leads to a covalent linkage of the indicator to the polymer without adversely influencing the quenching capacity of the indicator luminescence.
Experiments with other polymers containing acid amide groups, e.g. polyacrylamide, established that said indicators can also be introduced into these polymers by a contact with aqueous indicator solutions, whereupon they can be leached slowly. After radiation of the polymer with blue or ultraviolet light, MAC and lucigenine are not leached any more.
A preferred embodiment of the process according to the invention is characterized in that
a solution of the linear-chain hydrophile polymer is produced;
the solution is applied to a transparent carrier material and the solvent is allowed to evaporate, a film of the linear-chain hydrophile polymer being formed on the transparent carrier material;
the film is brought into contact with the solution of the luminescence indicator, the luminescence indicator diffusing into the film and optionally being immobilized by radiation.
Preferably, the thickness of the film is up to 20 xcexcm. By punching small disks out of the coated transparent carrier material, individual sensor elements are produced. In principle, this process step is known from the literature (M.J.P. Leiner, Optical Sensors for in vitro Blood Gas Analysis, Sensors and Actuators B 29, (1995) 169-173).
Another preferred embodiment of the process according to the invention is characterized in that
the solution of the luminescence indicator is brought into contact with a powdery linear-chain hydrophile polymer, the luminescence indicator diffusing into the linear-chain hydrophile polymer and optionally being immobilized by radiation;
the linear-chain hydrophile polymer carrying the luminescence indicator is suspended in a hydrogel solution, a suspension being formed;
the suspension is applied to a transparent carrier material; and
the solvent is allowed to evaporate from the hydrogel solution, wherein on the transparent carrier material there is formed a hydrogel film in which the linear-chain hydrophile polymer carrying the luminescence indicator is embedded.
Here, the linear-chain hydrophile polymer is preferably used with a particle size of less than 1 xcexcm. However, the particle size may also be up to 20 xcexcm.
The advantage of this procedure is that using aliquot portions of one and the same charge of the luminescence-indicator-covered powdery linear-chain hydrophile polymer for many sensor production charges allows large quantities of sensors having very uniform properties to be produced.
Then, from the carrier material thus coated, individual sensor elements can be produced by punching small disks.
The invention further relates to a process for the determination of the concentration of chloride in an aqueous sample while using the optical-chemical sensor according to the invention.